The present invention relates to teletext, and, more particularly, to a method and apparatus for displaying extended characters for refreshing teletext pages.
Teletext is defined, for example, in European Television Standard 300,706 dated May 1997, which is incorporated herein by reference in its entirety. Teletext is an auxiliary service provided by television channels for transmitting written information (text) in addition to video (images) and audio (sound) information.
The written information is encoded in digital form, and is transmitted in data packets at video frequencies at time intervals corresponding to frame synchronizations. A packet of teletext data, or a teletext packet, is thus sent every 20 milliseconds. The teletext packet is received and decoded by a teletext decoder incorporated within, or is connected to a television receiver. In other words, a teletext packet occupies the frequency band allocated to the television signal during a period corresponding to the period over which a scan line is transmitted.
A television screen can display a page of teletext having 25 lines and 40 columns, i.e., 25xc3x9740=1,000 displayed characters. Nevertheless, a page of teletext is described by more than 25 teletext packets. For a complete definition of a page of teletext, a much greater number of packets can be transmitted. Different type teletext packets are distinguished by a number ranging from 0 to 31. This is referred to as the Y number in the terminology referenced teletext standard.
There is thus transmitted a packet of a first type, referred to as a header packet, for which Y is 0, and which contains, in particular, information pertaining to the page number. Then, twenty five packets at most of a second type are transmitted, referred to as normal packets or display packets. Y is equal to 1 to 25, respectively, which the packets respectively contain level 1 (normal) characters for direct display on the television screen. Finally, depending on circumstances, packets of a third type referred to as non-displayable packets are transmitted. Y is equal to 26, 27, 28, 29, 30 or 31. The packets contain, among other, display enhancement data.
The non-displayable packets for which Y is equal to 26 are also referred to as X/26 packets. They contain, in particular, extended characters which are superimposed with the normal displayed characters. Extended characters are special characters, and include characters such as @, xcexc, #, and, $, £, . . . They are also called X/26 characters.
All of the teletext packets required to define all the teletext pages of the service are transmitted successively in cycles. The pages of a teletext service can be associated, if needed, to form logical sets called magazines. When the teletext service is called in, the teletext decoder automatically seeks page 100. Page 100 is a contents page in which are presented the different available magazines as well as the number of the first page of each of those magazines.
The contents page is loaded into a display memory of the teletext decoder. The contents page is read continually by the television""s display to display its contents on the screen. In other words, the screen page is at each moment the image of the teletext page stored in the display memory. When the user selects another teletext page, the contents page is loaded into the display memory and is subsequently displayed on the screen.
When a page of teletext is displayed on the screen, the data received during subsequent cycles in teletext packets which corresponds to that page are loaded into the display memory, either directly or indirectly. Two different modes for displaying teletext pages are known.
According to a first mode, the teletext pages are loaded into a buffer memory in step with the reception of teletext packets transmitted during a complete cycle. In order to display a given teletext page, the corresponding data are loaded from the buffer memory into the display memory. In other words, the data of the teletext page to be displayed pass through a buffer memory. This makes it possible to prevent undesirable display effects caused by the fact that not all the characters to be displayed are necessarily received at close enough intervals.
However, a buffer memory having a capacity on the order of 1 Kbyte per page is required to be stored. Such a buffer memory is penalizing with respect to the cost of televisions operating according to this display mode. This is why the second display mode is generally preferred. According to this second mode, the data of the teletext page to be displayed are loaded directly into the display memory in step with reception of teletext packets.
The expression xe2x80x9crefreshing the displayed pagexe2x80x9d is understood to mean replacing the characters of the displayed page with the characters defined by the teletext packets corresponding to that same page and received during a later cycle. The purpose of refreshing is to ensure that the display takes into account a possible change in the contents or in the presentation of that displayed page. It also makes it possible to obtain animation effects for the displayed page.
For both extended characters and normal characters, the order of reception of the characters is by convention. This convention is as follows, which considers the television screen as it appears to the user: from the top line to the bottom line, and within each line from left to right. In what follows, unless otherwise stated, the words first, last, previous, following, or similar are used with reference to this convention.
According to known display methods, the extended characters in the displayed page are erased globally for each line of the page upon reception and decoding of the first extended character to display in that line. As an illustration, consider the example of a text initially displayed on a given line as shown in FIG. 1a. This line gives the address of an imaginary electronic mailbox of an imaginary company called TOTOandCie. The text for this line includes two extended characters, which are and and @.
The first step in the process of refreshing a line according to the prior art includes erasing the two extended characters and and @ so that the line on the screen appears as shown in FIG. 1b. In this figure, the position of the erased characters is represented symbolically by a dash in the underscore position. However, a space actually appears at these positions on the screen according to the user.
At a second stage, the character and is displayed further to receiving and decoding the teletext packet in which it is encoded and transmitted. The line then appears to the user as shown in FIG. 1c. At a third stage, the character @ is displayed further to receiving and decoding the teletext packet in which it is encoded and transmitted. The line then appears to the user as shown in FIG. 1d. 
A problem arises from the fact that the X/26 packets in which the characters and and @ are encoded and transmitted are not necessarily the same packet, nor even two consecutive X/26 packets. As a consequence, the time interval between the first stage and the second stage, and/or between the first stage and the third stage, can exceed the retinal remanence of the eye. Retinal remanence is the property of the human eye not to perceive variations in an image which occur at a frequency exceeding a certain threshold. The resulting visual effect to the user is that of a flicker. This display effect is undesirable to the user.
Such a problem only arises when refreshing a previously displayed teletext page. When the user selects a new teletext page, the display memory is erased and the data of the new teletext page to be displayed are loaded in step with the reception of corresponding teletext packets. Accordingly, there occurs no undesirable display effect. This also applies when displaying the first teletext page (contents page) upon entering the teletext service. As mentioned above, such a problem does not arise with teletext devices having a buffer memory.
An object of the present invention is to solve a problem specific to the use of teletext decoders operating according to the above described second display mode. This problem arises upon refreshing the displayed page, and is inherent to known methods of displaying extended characters for refreshing a teletext page, as implemented in an apparatus that does not have a buffer memory.
This and other objects, features and advantages in accordance with the present invention are provided by a method for displaying extended characters for refreshing a teletext page, which comprises successively for each of the extended characters to be displayed in a determined line of the teletext page the following steps:
a) erasing the old extended characters present in the line between the display position of the new extended character to be displayed and a determined position further back, the latter not included. The determined position further back is the display position of the new extended character preceding, in the determined line, the extended character to be displayed or, failing that, the display position of the first character of the line; and
b) displaying the extended character to be displayed at its display position.
In this way, when refreshing a teletext page, the extended characters are only erased which, as the case arises, must disappear. In this way, no flickering effect is observable by the user.